L. Cesnak

Relation between critical current and exponent n in Bi(2223)/Ag tapes

Transport critical currents at and exponents n of the power law characteristics of single-filament Bi(2223)/Ag tapes were measured at 77 K in the self-field and in external magnetic fields up to 1.2 T. The effects of tape orientation in a transverse magnetic field from to (related to the basic state at -axis) were investigated as well. The values of the transport critical current and exponent n as well their mutual relation reflect the quality of the Josephson-linked current network connectivity at ( is the cross-over field at which the weak-link currents cease to contribute to the transport current) and the ability of pinning centres to pin flux lines in the whole magnetic field region. The weak-link connectivity and the pinning potential are directly related to the structural state of the samples resulting from their different thermomechanical treatments. The better the structural quality, the higher the ratio. This ratio seems to reflect the effective number of intergrain connections and pinning centres.

Magnetic field stability of superconducting magnets

Abstract A simple qualitative model is proposed to explain the fast initial change of the magnetic field of superconducting magnets which operate in the persistent mode. Using this model the additional component of the transport current which is induced by the decaying screening currents can be represented. The analytical results are in good qualitive agreement with the experimental curves. By a suitable cyclic approach to the frozen-in current it is possible to attain very good stability from the start.

Bending of Bi(2223) - Ag tapes at 77 and 300 K

An experimental study of Bi(2223) - Ag tapes undergoing bending and straightening at 300 and 77 K has been performed. The special test equipment allows us to measure changes of the critical current in a short element of the tape during continual decreasing or increasing of the bending diameter at both temperatures mentioned above. Comparison of 300 and 77 K results shows the effect of the Ag sheath strength on the bending sensitivity of Bi(2223) - Ag composite tape. Remarkable lowering of critical current degradation by bending at 77 K was observed. This offers the possibility of winding small-diameter coils with Bi(2223) - Ag tape pre-cooled by liquid nitrogen.

Pinning potential, its relativity and dependence on temperature and magnetic field studied on the basis of the I - V characteristics of multifilamentary superconductors

A series of I - V characteristics of multifilamentary superconductors measured at various temperatures (2.4 - 4.2 K) and magnetic fields (6 - 12 T) was analysed with the help of creep process theory. It was shown that the current-dependent effective pinning potential as well as the temperature- and magnetic field-dependent pinning potential depend on the choice of the reference current - voltage point in some of the currently used models. Due to this fact, the values of as well as derived from I - V data have a meaning related to a reference current - voltage point chosen usually in the practically accessible measurement range. In this paper, we used 1 as the reference voltage. Generalizing some features of creep models we found the way to derive from experimental data without knowing the precise form of the dependence. The influence of technological parameters (filament composition and heat treatment conditions) on this relative pinning potential are discussed. A simple scaling of was found for all the conductors under study. Its form is , where the values of scaling temperatures are very close to the critical temperatures and seems to be a maximum value of the relative pinning potential at a given magnetic field in the whole temperature range.

Treating the I-V characteristics of low as well as high Tc superconductors in context with the pinning potential

Abstract A method is presented for treating the I-V characteristics of superconductors in the flux creep regime in context with the effective pinning potential and its dependence on the transport current. Some common features are shown in the behavior of different (low, as well as high Tc) superconductors. This common behavior manifests itself in the very similar form of the reduced apparent effective pinning potential, U efa U 0a wnen plotted against the logarithm of the reduced current I I 0 . The apparent pinning potential, U0a and some reference current, I0 are used as scaling quantities. The analysis concerns two samples of multifilamentary low Tc superconductors NbTi and Nb3Sn, as well as two laboratory samples of high Tc YBa2Cu3O7-δ superconductors. The main result of the presented analysis is that the plot of U efa U 0a vs ln( I I 0 ) was found to be close to a straight line having a common negative slope d ( U efa U 0a ) d(ln( I I 0 )) = − 1 or all studied samples disregarded from the material and from the value of temperature and magnetic field. Some slight deviation from a straight line indicates a digression from the power law V≈In characteristics as shown in the case of the YBa2Cu3O7−δ material.

Ac losses in multilayer superconducting tapes

Abstract ac losses have been investigated experimentally as well as theoretically in tapes having on the surface several normal conducting and/or superconducting layers. The superconducting layers under investigation have been Nb 3 Sn on a niobium substrate and Nb 3 Ge on a stainless steel substrate. It has been proved that the layered structure of the tapes is well reflected by the stepwise character of the ac losses dependence on the amplitude of the surface magnetic field. The magnetic flux passing through a surperconducting layer or surface barrier into the inside of the tape enhances the losses in the passed barrier or layer.

Equivalent scheme for the V - I characteristics of Bi-2223/Ag tapes at 77 K

A series-parallel equivalent scheme is proposed to obtain better understanding of the course of experimentally observed V-I characteristics of multifilament Bi-2223/Ag tapes at liquid nitrogen temperatures. The elements of the equivalent scheme reflect the intrinsic properties of the intergranular, respectively of the intragranular parts of the composite superconductor. The dependence on the transversal magnetic field as well on the relative orientation of the magnetic field with respect to the tapes broad face of two samples is presented and compared. The proposed method could offer better insight into the inner structure of the studied samples.

Sample holder for measuring of -anisotropy in high magnetic fields

The sample holder for the measurement of angular dependencies of I c (B) characteristics of superconductors in high magnetic fields generated by a superconducting solenoid magnet is described. The examples of application for flattened bronze route Nb 3 Sn and powder in tube method Bi(2223)/Ag tapes are presented.

Ic anisotropy in flattened Nb3Sn superconductors and possible ways for overcoming it

Abstract In bronze-route processed and flattened Nb 3 Sn multifilamentary superconductors under reasonable heat treatment conditions, the critical current in a parallel magnetic field with respect to the flat side of the conductor is lower than the critical current in a perpendicular orientation. This feature is unfavourable in terms of using such conductors in coils, where the highest magnetic field in the coil central plane is parallel to the conductor wound on its flat face (in the R&W technique). In this paper, the main features of the I c anisotropy problem are summarized and two possible ways to overcome it are proposed, involving the construction of a flat cable and a combined conductor using multifilamentary Nb 3 Sn tapes.

Influence of conductor temperature on the real voltage-current characteristic of composite superconductors

An experimental study of voltage-current characteristics performed on a special Nb3Sn multifilamentary superconductor revealed that the increase in conductor temperature during the test influenced the shape of the characteristics. A theoretical solution of the problem is presented. As a starting supposition the validity of the intrinsic isothermal power law E approximately In over a wide range of voltages is accepted. The real E-I points on the characteristics are determined by the thermal equilibrium in the conductor given by losses and cooling.